A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned.
Lithography is widely recognized as one of the key steps in the manufacture of ICs and other devices and/or structures. However, as the dimensions of features made using lithography become smaller, lithography is becoming a more critical factor for enabling miniature IC or other devices and/or structures to be manufactured.
Convention lithographic apparatus use radiation having a wavelength of 193 nm. This is an example of deep ultraviolet (DUV) radiation. In order to shorten the exposure wavelength and, thus, reduce the minimum printable size, it has been proposed to use an extreme ultraviolet (EUV) radiation source. EUV radiation is electromagnetic radiation having a wavelength within the range of 5-20 nm, for example within the range of 13-14 nm. It has further been proposed that EUV radiation with a wavelength of less than 10 nm could be used, for example within the range of 5-10 nm such as 6.7 nm or 6.8 nm. Such radiation is termed extreme ultraviolet radiation or soft x-ray radiation. Possible sources include, for example, laser-produced plasma sources, discharge plasma sources, or sources based on synchrotron radiation provided by an electron storage ring.
Various types of radiation sensors may be used as metrology tools. For example they may be used to characterize optical elements in a processing tool such as a lithographic apparatus, providing information regarding the quality of the image. An example of such sensor is a CMOS type, front-side illuminated, interferometer sensor which typically uses a conversion material (i.e. a scintillator) to convert (D)UV or EUV photons to photons with longer wavelengths, and then detect the longer wavelength photons. Such conversion material based sensors may suffer however from poor resolution and/or poor signal to noise ratios, they are slow and may be blurry, or may suffer from other disadvantages.
Other sensors were proposed in the prior art based on direct radiation detection (with no use of a conversion material). One example of direct radiation detection sensor is described in US 2004/0021061 A1, which relates to a charge-coupled (CCD) backside-illuminated sensor having a n-on-p junction photodiode. Alternative protective layers including boron are proposed instead of standard SiO2 passivation layer to avoid oxidation or contamination of the sensor surface. However, even a few nm thick protective layer may be sufficient to provide absorption of radiation, therefore an improved sensor stability may be achieved at the cost of decreased sensitivity.
Another variant of a direct radiation detecting sensor is described in U.S. Pat. No. 5,376,810 wherein a backside-illuminated CCD uses a low temperature (≦450° C.), delta-doping molecular beam epitaxy (MBE) process to grow a sharp dopant profile of a few atomic layers in silicon. Such sensor seems to be however limited to UV spectrum only.
A further direct radiation detecting sensor is described in EP 2009705, wherein a p-on-n junction is created at the front surface of a semiconductor substrate by depositing a p-dopant (boron) material on top of a n-doped semiconductor using a high temperature deposition process such as chemical vapor deposition (CVD). Such front-side illuminated sensor provides high sensitivity for EUV and (D)UV radiation and good surface charge collection efficiency, as also shown in chapter 4 of PhD thesis entitled “Performance Analysis of Si-Based Ultra-Shallow Junction Photodiodes for UV Radiation Detection”, by Lei Shi (April 2013). Although such high-temperature boron deposition technology may be suitable for manufacturing imaging sensors with a simple construction, such as transmission imaging sensors, it has been found unsuitable for manufacturing CMOS or CCD based radiation sensors because components of the sensor such as internal circuitry, wiring or poly-silicon gates are damaged.